Racemization of lysine



Jan.- 2, 1 9 51 naosmza'nos or stem lobertlhlmmiokandllenneth odi'ambrookr Niagara hit, and Arthur 0. Rogers, Iiewiston. N. Y assignors to E. I. dnl'ontdoNemours l Company, Wllmlngton, Del a corporation of Delaware No Drawing. Applies Serial 1 m invention relates to the racesnisation of amino acids and more particularly it relates to a novel process for the racemization of lysine.

Lysine is one of the amino acids which occur in nature as constituents of proteins. The lysine which occurs naturally is optically active, its aqueous solutions rotating the plane of polarized light to the right and this form is commonly designated as l(+)-lysine. In this form lysine is an essential component of animal diets whereas the enantiomorphic d( -)-1ysine has no known nutritional value.

Lysine which has been synthesized from optieally inactive materials, for example. by the process of Eek and Marvel, Organic Syntheses Collective Vol. II. page 374, or by the process described in application 8. N. 70.799 filed January 13. 1949 is optically inactive and consists of equal parts of the biologically active l(+)-isomer and the biologically inactive d() -isomer. Thus. its nutritional value can be doubled by converting its d() -lysine content into the l(+) -form while retaining the quantity of the latter initially present. 7

It is possible to convert synthetic lysine completely to the biologically active form by first separating the isomers (resolution) and then converting the inactive d()-lysine back to the dl-mixture. (racemization). By a repetition of these operations, the lysine is eventually converted entirely into the l(+)-form. As a'practical matter. it is unnecessary and usually impossible to effect a complete separation of the l(+) -lysine and the d() -isomer. The requirements for an economically feasible process are that a substantial portion of the l(+)-lysine be isolated in pure form in the resolution step and that no serious loss of total lysine occur in either step.

Resolution of dl-lysine is usually eflected by combining the material to be resolved with an optically active compound known as the resolving agent and fractionally crystallizing the product. This may be accomplished by known methods. For example, in a method disclosed by C. P. Berg in the Journal of Biological Chemistry, vol. 115, pages 9-15, 1936, d-camphoric acid is used as a resolving agent to obtain l(+)- lysine. This author also shows the use of l-camphoric acid to obtain d() -lysine.

The racemization of lysine, or conversion of the optically active material to the racemic mixtures, has previously been accomplished by heating the isomer with hydrochloric acid. This method s disclosed in Ber. so. me, nann thu' November as. im. misuse 0 Claims. (c1. see-m) 2 Although good recoveries of lysine are obtainable by this method. the mixture of hydrochloric acid and lysine is highly corrosive to metals, particularly at the temperatures necessary to eifect raoemization at a practical rate. Thus. expensive corrosion resistant equipment is necessary if this method is to be used. Attemptsto use this method by reducing the proportion .of hydrochloric acid to that stoichiometrically equivalent -to the lysine present have failed to obviate this serious disadvantage. The use of hydrochloric acid also results in a complicated process since it is first necessary to replace the resolving agent with the hydrochloric acid prior to racemiaation and following the racemization step it is to reconvert the lysine to the free form before it can be recombined with the resolving agent in the next operation. Another method for the racemization of lysine which is go disclosed in U. S. P. 2,071,327 consists in heating the amino acid with acetyl chloride in the presence of acetic acid. Obviously, this method is even less desirable than the one described above.

It is an object of this invention to provide a as simple, practical and economical process for the racemization of lysine. Another object is to provide a process for the racemization of lysine wherein the operation may be carried out under non-corrosive conditions permitting the use of ordinary metal equipment. A further object is to provide a new and improved process by which a mixture of l(+) and d()-isomers of lysine may easily be recovered from the reaction medium. Still another object of the invention is to provide a new process wherein a mixture of l(+) and d()-isomersof lysine may be resolved. the undesired isomer racemized in accordance with the process described herein and the resulting mixture recycled direct to the resolution step. These and other objects will be apparent from the ensuing description of the invention.

The above objects are attained in accordance with this invention by heating optically active lysine in combination with .a cation-exchange material.

In a preferred mode of operating the invention, an optically active lysine is heated in combination with a cation-exchange material in the presence of water.

In general. the process of this invention is carried out by passing an aqueous solution of optically active lysine which may be in the form of free lysine. a salt of lysine with a resolving agent, or other suitable form into a cation-exchange material and heating to the desired temperature until racemization is accomplished.

vary with the temperature as well as the ratio of optical isomers in the lysine treated and usually a few minutes to 48 hours is suilicient. It is prefen-ed to carry out the racemization by operating within theme 100' C. to 230 C. A temperature of 150 C. has been found entirely satisfactory. Although higher temperatures may be utilized if desired, relatively heavy equipment is necessary to withstand the pressure developed and some decomposition of lysine and the cationexchange material may occur. Temperatures below 100 C. may be utilized, but the reaction is slow and therefore requires larger reaction vessels for a given production rate.

Although racemization may be carried to completion in accordance with this invention it is unnecessary and may be undesirable in practical operation to racemize completely. Since mixtures containing varying proportions of l(+)- lysinse and d( -lysine may be resolved without diliiculty it is entirely satisfactory to stop the racemization at any' desired point. However, it is of course desirable to increase substantially the proportion of the desired isomer in the mixture.

Although lysine may be racemized by heating in combination with the cation-exchange material in the absence of water, it is preferred to operate in the presence of water to avoid loss of lysine through dehydration reactions.

Any of the well-known cation-exchange materials may be utilized in the practice of this invention. As commonly used the term cation-exchange materials refers to a class of insoluble polymeric materials which contain acidic groups capable of combining with a variety of cations to form insoluble salts. A typical example of a cation-exchange material is described in U. S. P. 2,366,007. Other examples include polyphenolsulfonic acids, sulfonated hydrocarbon polymers and polycarboxylic acids. Sulfonated coals and natural and synthetic zeolites having cationexchange properties are also suitable in the practice of this invention. The selection of a particular cation-exchange material will depend upon such considerations as capacity, exchange rate, mechanical ruggedness and chemical stability as well as cost. Any of the commercially available cation-exchange materials are suitable in the practice of this invention. For example, satisfactory results have been obtained with cation-exchange materials sold under the names Dowex-50, Amberlite IR-100, Amberlite IR- 105," Amberlite IR-120, Ionac -200, "Duolite C-3, Amberlite IRC-50 and Permutit Q.

A preferred class of cation-exchange materials are those in which the acidic groups are sulfonic acid groups as exemplified by the sulfonated hydrocarbon polymers. These cationexchange materials are particularly valuable in the practice of this invention since they possess excellent chamical and physical stability under the conditions utilized, are readily available, and have high capacity and exchange rate.

The following examples are illustrative of the invention.

Example 1 One-half mole (246 g.) of lysine d-camphorate in which the ratio of d() -lysine to l(+) -lysine was 64:36 ,was passed through a 1.5-liter bed of a commercial cation-exchange resin comprising a sulphonated polymerizerate of a polyvinyl aryl compound in its ammonium form. After washing, the resin-lysine combination was heated for 18 hours at 149 C. in a pressure-tight vessel. The resin was eluted with 10% ammonia and the eiliuent boiled to expel the excess ammonia. Free lysine recovered was equivalent to 89% of that originally fed to the resin bed and to virtually all of that retained by the resin during the charging operation. The ratio of d()-lysine to l(+)-lysine in the product was 52:48.

Example 2 L(+)-Lysine Heating L(+)Lyslnc I before trcat- 'Iimc, after treatmont Hrs. mom, per cent 32 16 41 32 40 130 46 3G 18 H9 48 3?. l 0. 5 M497 ,4!)

50% l(+)-lysine content corresponds to complete racemimtion.

Example 3 The efiectiveness of representative cationexchange resins in promoting the racemization of lysine was tested in the following experiment. In each case 50 cc. of the resin in ammonium form was charged with 200 cc. of a solution containing lysine combined with resolving agent, in which the ratio of l(+) -lysine to d(-) -lysine was 18.5:81.5. After washing, the charged resins were sealed inglass pressure tubes and heated 16 hours at C. The lysine from each was then eluted with 10% ammonia, the excess ammonia-boiled off and the percentage of l(+)- lysine in the recovered material determined. It should be noted that capacities and optimum flow rates for the different resins were not determined, and hence that the recoveries of lysine were incomplete in most cases.

Lysine con- Lysine Recovered tent of Charging Solution 0. Per

1 50% l(+) content indicates complete racemization.

In this case thedlflvlsine was recovered partially as free lysine and partially as lysine ydrochloride.

I Six diflcrent commercially available cation-exchange materials.

Example 4 Two hundred forty-six grams of lysine d-camphorate in which the ratio of d(--)-lysine to l(+) -lysine was 64:36 was dissolved in 1 gallon of water-and passed slowly through a bed of cation-exchange resin in ammonium form. The

at the desired temperature in the ing the resolving. agent ter for seventeen hours. After cooling the resin was removed from the pressure vessel and eluted with 3 liters of ammonia. The elutriate was evaporated and the recovery of lysine found to be 89.4% of theoretical. The ratio of 1(+) -lysine to d() -lysine in the product was 48:52. The product was combined with 89 g. of d-camphoric acid and resolved by fractional crystallization from 50% aqueous methanol. The crystals thus obtained weighed 54.3 g., yield of 1(+)-lysine d-camphorate; the optical purity of the product was 94%.

Although the process of this invention isuseful in racemizing optically active lysine from any source, it is of particular utility as a part of the resolution-racemization cycle involved in converting dl-lysine or other mixture of 1(+) and d( -lysine to a pure optical isomer. This procedure which may vary in detail consists essentially in resolving lysine by known methods; for example, the method described by Berg, separatingv the desired isomer, heating the remaining undesired isomer in contact with the cation-exchange material to effect racemization, recovering a mixture of 1(+) and d(-)-lysine and returning the latter to the resolution step; An example of this mode of operation follows.

A mixture of,1(+) and d(- )-lysine is resolved with d-camphoric acid and the 1(+)- isomer is crystallized out and separated from the mother liquor. The mother liquor may contain water, methanol and salts of the resolving agent with lysine in which the d()-isomer of the latter predominates. The mother liquor is diluted with water and passed through a bed of cation-exchange material preferably in the ammonium form. During this passage and subsequent washing with water, the camphoric acid passes into the diluent in combination with ammonia and may be recovered for reuse. The d()-lysine is held in combination with the cation-exchange material. The cation-exchange material with the combined lysine is then heated presence of water to racemize the lysine. The cation-exchange material is then eluted with aqueous ammonia which displaces the lysine and reconverts the cation-exchange material to its ammonium term. The emuent containing ammonia and lysine is partially evaporated to remove ammonia and the residual solution or a mixture of 1(+) and d()-lysine is returned to the reao lution step. In the process described above it is preferred to utilize the cation-exchange material initially in its ammonium form or Otherwise. dimcuity may be encountered in washcompletely from the cation-exchange material. If desired, the amine,

representing a 52% other form capable of V producing soluble salts of the resolving agent.

. methods for the scale commercial operation. It is particularly valuable in connection with recently developed production of synthetic lysine which is potentially of major importance as a component of animal feed. i

We claim:

1. Process for the racemization of lysine which comprises heating optically active lysine in combination with a cation-exchange material.

2. Process for the racemization of lysine which comprises heating optically active lysine in combination with a cation-exchange material in the presence of water.

3. The process of claim 2 wherein the temperature is maintained at IOU-230 C.

4. The process of claim 3 wherein the acidic groups of the cation-exchange material are sulfonic acid groups.

5. Process which comprises resolving a mixture of 1(+) and d()-lysine. separating a desired optically active isomer, heating the remaining undesired optically active isomer in combination with a cation-exchange material and recovering a mixture of 1(+) and d(i-lysine.

6. Process which comprises resolving a mixture 01' 1(+) and d(-)-lysine into 1(+)-lysine and d(-) -lysine, separating 1(+)-lysine, passing an aqueous solution of the remaining d(l-lysine into contact with a cation-exchange material, heating said d()-lysine in combination with said material in the presence of water and recovering a mixture of 1(+) and d()-lysine.

7. The process of claim 6 wherein the acidic groups of the cation-exchange material are sulmonia, and recovering a mixture of 1(+) and d() 4mm from the elutriate.

ROBERT D. EMMICK. KENNETH O. ARTHUR 0. ROGERS.

No references cited. 

1. PROCESS FOR THE RACEMIZATION OF LYSINE WHICH COMPRISING HEATING OPTICALLY ACTIVE LYSINE IN COMBINATION WITH A CATION-EXCHANGE MATERIAL. 